1. Field of the Invention
The present invention relates to a device for demodulating digital data included in RDS (Radio Data System) broadcast signals or FM (Frequency Modulated) multiplex broadcast signals and, in particular, relates to the generation of reliability data required for performing soft decision error corrections on demodulated data.
2. Description of the Prior Art
In general, a received signal contains substantial noise components due to the influence of phasing and so on occurring over transmission lines, which may make it impossible for a demodulator to accurately decide whether the received signal is 1 or 0. Conventionally, by means of comparison with a suitable threshold value, the decision as to 0 or 1 has been made for demodulation so that the signal may be input to subsequent synchronous regenerator or error correction circuits. The accuracy of a receiver system may be improved by outputting data representing the reliability of a demodulated digital signal (hereinafter referred to as reliability data) anew in accordance with the level of a received signal and using such reliability data for error corrections and so on at subsequent stages. A related art for such a soft decision error correction scheme has been proposed by the Applicant in Japanese Patent Application No. Hei 7-134511.
However, such soft decision error corrections have not been made in conventional RDS decoders. A common method for generating reliability data has been performed as described below.
For an input signal having multiplexed digital information ((a) in FIG. 1), a sampling pulse (b) is regenerated by a clock regenerator circuit in synchronism with the data, and the input signal (a) is sampled on the basis of the sampling pulse (b) and thereby converted to digital data (A/D conversion) (Refer to (c)). Conventional data (that is, hard decision data) is determined according to whether an input signal level is higher or lower than a threshold level Vt at a sampling timing and the MSB of the A/D conversion applies as it is. On the other hand, the reliability data for soft decision error correction is determined according to whether the distance of the input signal waveform at the sampling timing from the threshold level Vt is large or small. It means that the reliability data will be larger (higher reliability) if the input signal is far away from Vt at the sampling timing, and will be smaller (lower reliability) if it is closer to it. An example of reliability data thus determined will be shown in (d).
For an RDS decoder, the above input signal (a) would correspond to BPSK (Binary Phase Shift Keying) demodulated output of an RDS signal extracted from a FM composite signal through a band pass filter (BPF). As will be described with reference to FIG. 2, in the absence of an ARI (Automatic Radio Information) signal, an RDS signal will have a waveform as shown in (a) and the signal (a) will be demodulated by a carrier signal (b) carrier regenerated in a demodulator. Although, for illustrative purposes, a signal having six carriers per symbol is shown here, there are actually 24 carriers in an RDS.
When the input signal (a) is A/D converted at a 90 degree phase of each cycle of the carrier (b) for example, the signal will be converted to digital data (c). By detecting the biphase timing as shown in (d) (which corresponds to (b) in FIG. 1) using a data clock regenerator circuit and using the value of the digital data (c) at this timing, the 0/1 decision of the received data (hard decision) and the detection of the reliability data may be made.
As described above, in order to acquire the reliability data, it is necessary to decide the waveform level of the input signal and, in order to know the waveform level of the input signal at a symbol data sampling timing, it is necessary to perform an A/D conversion. For this reason, when an input signal level varies due to a weak electric field and so on, such level variance will be directly reflected in the reliability data. To avoid this, it has been essential to perform an accurate AGC operation, with a result that the circuit disadvantageously tends to be more complicated. Especially for an RDS signal, this problem is more serious because when an ARI signal is multiplexed, the amplitude of an RDS/ARI multiplex signal may vary about five-fold at maximum depending on the modulation factor of the ARI signal.
Again for an RDS signal, RDS source data has been differentially coded and the differentially coded data has been coded into a biphase symbol. Such a biphase symbol represents data by a set of two symbols. For example, even if one of the symbols failed to be decoded correctly, but the other was decoded with a high reliability, it would be possible to regenerate the data correctly as long as the pair of the biphase is decided correctly. If hard decision data and reliability are to be determined for each symbol as in the conventional, however, this feature of the biphase code may not be actively utilized.